pneumatophore

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A gas-filled sac serving as a float in some colonial marine hydrozoans, such as the Portuguese man-of-war.
• noun Botany A specialized respiratory root in certain aquatic plants, such as the bald cypress, that grows upward and protrudes above the water or mud into the air.

from The Century Dictionary.

• noun A pneumatocyst, or a structure which supports such a float; especially, the proximal dilatation of the cœnosarc or hydrosome of the Physophoridæ See cuts under Athorybia and Hydroszoa.
• noun An apparatus for supplying a person with oxygen when in a place filled with bad or unwholesome air.
• noun In phytogeography, a lateral root, submerged or more often exposed to the air, at least periodically, so differentiated as to serve the purpose of respiration; a respiratory root.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Zoöl.) One of the Pneumonophora.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun zoology A gas-filled sac or float of some colonial marine coelenterates, such as the Portuguese man-of-war.
• noun botany An aerial root, in mangroves etc., specialized for gaseous exchange.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun an air-filled root (submerged or exposed) that can function as a respiratory organ of a marsh or swamp plant